Ethanol producers often purchase yeast for fermentation from a yeast supplier (e.g., in dried form, and/or as a cream liquid suspension). Yeast can constitute a large operational cost for many ethanol producers, and as such many ethanol producers add a bare minimum of yeast to fermentation processes.
However, there may be a number of benefits to increasing yeast loading within a fermentation. For example, one substantial benefit can be a reduced time period for fermentation, thereby effectively increasing the output for a given ethanol producing facility. Another major benefit of increased yeast loading can be a reduction in losses relating to lactic bacteria contamination. A higher yeast loading can mean that contaminants cannot compete with the dominant yeast population, thus ensuring a cleaner operation. Unfortunately, given the cost of yeast, increased inoculation of fermentations is often cost prohibitive.
If ethanol production facilities could grow their own yeast in a cost effective manner, then larger doses of yeast could be added to the fermenters. This can result in multiple benefits, including reduced yeast costs, faster fermentation (effectively increasing plant capacity), and/or reduced microbial contamination risks (due to out-competition of lactic bacteria by the larger yeast population).
However, growing (propagating) yeast such as, e.g., Saccharomyces cerevisiae can be challenging. For example, if the yeast is grown in too high a concentration of glucose, the yeast can switch over from aerobic metabolic pathways to ethanol producing anaerobic metabolism, even under highly aerated conditions. This shift, when propagating yeast is generally not desirable if the purpose is to generate substantial numbers of yeast cells. Even under highly aerated conditions, if the glucose concentration in a propagation medium exceeds about 5 g/L, the yeast, S. cerevisiae, can sometimes begin to make ethanol (fermentative pathway). This is known as the “Crabtree” effect (suppression of respiration by high glucose). Likewise, when not enough oxygen is present, metabolism may shift to the fermentative pathway.
To help avoid the Crabtree Effect, Saccharomyces cerevisiae yeast is often grown by yeast suppliers in well aerated yeast propagation tanks with tightly monitored glucose feed (typically molasses feedstock is used in a fed-batch process) to help ensure that glucose levels remain low enough that metabolism remains aerobic.
Unfortunately, for the vast majority of ethanol producers, maintaining the conditions necessary to grow their own yeast can be economically and/or technically infeasible. The equipment and technical expertise required to generate yeast in appreciable volumes is often too great a risk and cost, and as such they must often rely upon yeast suppliers. For example, careful metering of the glucose stream by an ethanol producer can be technically difficult due to large batch-to-batch variation in molasses, and raises a major risk if done improperly due to facility shutdowns associated with a shortage of yeast. Further, molasses is not typically utilized in an ethanol production facility the United States (North America), and the logistics required to have molasses delivered would often be a major hurdle to growing yeast by an ethanol producer.
It would be advantageous to provide for alternative systems and methods to propagate yeast, e.g., systems and methods that do not require stringent molasses metering systems. Such systems and methods could allow for more preferred yeast dosing in ethanol fermentations.